The present invention relates to the type of time color indicator described in U.S. Pat. No. 4,212,153 issued to Kydonieus et al entitled "Time Color Indicator". In general, the Kydonieus patent discloses a time color indicator having a reservoir layer adhesively attached to an indicator layer. In accordance with the Kydonieus patent, a migrating agent in the reservoir layer migrates through the adhesive layer and the indicator layer to the top or front surface of the indicator layer. The Kydonieus patent also teaches that the reservoir layer is preferably mounted on a barrier layer with an adhesive and release sheet on top of the reservoir layer. The indicator is activated by removing the release sheet and applying an indicator layer to the adhesive layer. Alternatively, the adhesive and release sheet may be associated with the indicator layer. Kydonieus further teaches that the indicator layer is a solid sheet or film of non-porous polymer which allows migration of the chosen agent, and that appropriate indicator layer materials include plasticized PVC, semi-plasticized PVC, rigid PVC, acrylics, polyurethanes and hytrel. The Kydoniesus indicator layer is 2 to 14 mils thick, and may contain plasticizers and stabilizers. The reservoir layer is preferably made from a plastisol, although vinyl chloride/vinyl acetate copolymer, a urethane polymer, a polyolefin, hytrel, and polyvinyl chloride may be used in forming the reservoir layer. Kydonieus teaches that the reservoir layer should be 1 to 20 mils thick, and is preferably 1 to 5 mils thick. Kydonieus also contemplates that the reservoir layer may be layered down as an ink onto the barrier layer. Specifically, Kydonieus contemplates printing the reservoir composition on mylar or aluminum foil. The Kydonieus patent suggests that appropriate adhesives include thermosetting or thermoplastic pressure-sensitive acrylics or rubbers. The adhesive layer can be preformed and applied as a sheet, applied with another layer such as a release sheet or by coating. One suggested application for the Kydonieus device is greeting cards.
Kydonieus further teaches that the amount of time required from initiation to perception depends upon and may be controlled through adjustment of the thickness of the indicator layer, indicator layer identity, reservoir concentration, interposition of barrier layers and/or selection of the migrating agent. Kydonieus also contemplates a white indicator layer to provide increased contrast.
Kydonieus discusses use of the time color indicator through comparison of the indicator with a color scaled to determine when conversion is complete. However, Kydonieus also suggests that the reservoir layer could be printed in the shape of words or using a stencilled barrier layer. In practice, ink containing the migrating agent has been printed in the shape of words, with the surrounding area overprinted with similar ink not containing the migrating agent. Typically, however, the screened message can readily be discerned notwithstanding the surrounding overprint.
While the Kydonieus Time Color Indicator represents a significant and useful advance in the art, providing reliable and repeatable control of the time and extent of change have proven difficult.
By way of example, it has proven difficult to provide a time color indicator which obtains consistent conversion in a matter of minutes or hours. For such a short duration indicator adjustment of the time control parameters discussed by Kydonieus does not provide adequate time control. Indeed, adjusting the indicator layer thickness, reservoir concentration, etc. in the manner suggested by Kydonieus have not been found to obtain reliable results. To the contrary, it has been found that the same construction could yield varying time change characteristics. Despite attempts to control the various parameters discussed in the Kydonieus patent, consistent results have been difficult to obtain.
Another problem is that Kydonieus structure is a two part construction, requiring the user to assemble the indicator and reservoir layers. In Kydonieus' preferred embodiment, a release sheet is removed from either the indicator layer or the reservoir layer to reveal a pressure sensitive adhesive for assembling the components. This construction is also relatively expensive to make, due to the fact that Kydonieus' two part construction requires that each part be produced in separate processes to be assembled in yet another step. The added costs incurred in such manufacture may make the time color indicator too costly under some circumstances.
In practice, the Kydonieus structure has been practiced with an indicator layer consisting of a solid layer containing titanium dioxide coated with a pressure sensitive adhesive and a release sheet. The two part construction is activated by removing the release sheet and applying the adhesive-bearing indicator layer to the reservoir layer. The migrating agent then travels through the pressure sensitive adhesive into the receptive titanium dioxide containing layer.
In longer term applications, it has been found that in the above-described structure the migrating agent or dye becomes dissolved or dispersed in the titanium dioxide containing layer, reducing the sharpness and effectiveness of the displayed message. To overcome this particular problem, it has been suggested to add an additional layer atop the indicator layer, including a further pressure sensitive adhesive layer and a clear, impermeable barrier. In this manner, the migrating agent travels from the reservoir layer through the titanium dioxide-containing layer into the additional pressure sensitive adhesive layer. The message to be displayed is clearly visible against the titanium dioxide background, but the migrating agent does not mix with, dilute, or become dissolved in the titanium dioxide layer. The structure including additional pressure sensitive and clear layers has recently been introduced to the market.
Therefore, it is one object of the invention to provide a time color indicator having improved time change characteristics.
It is a further object of the invention to provide a time color indicator having reliable short duration time change characteristics.
Another object of the invention is to provide a time color indicator which is economical to make and easy to use.
Yet a further object of the present invention is to provide a time color indicator that may readily be activated in a one-step activation procedure.